Determining the position of products against a background or a substrate can be an aspect of major significance whenever, in a production procedure, products which are positioned in an unregulated or random fashion are to be handled automatically, for example more particularly but not exclusively when packaging products.
A typical situation of use in this respect involves a background or substrate in the form of a conveyor belt, by means of which the individual products that have been produced are conveyed out of a production installation to a packaging station where they have to be automatically gripped, for example by means of suitable gripper devices or other handling devices, and transposed into packaging containers, for example trays, which are ready to receive them in pockets which are generally pre-shaped therein.
A difficulty in this respect is that the individual products are generally disposed on the conveyor belt in positions which cannot be determined beforehand and also point in different directions, thus involving different directional orientations. In contrast, the products have to be deposited in the packaging containers or trays not only in the correct respective positions relative thereto but also in a given positional orientation, depending on the respective product involved and the configuration of the respective tray or the pockets in the tray. Furthermore that situation is often made more difficult by virtue of the fact that a plurality of different kinds of products may be supplied in a randomly distributed arrangement on the product-carrying conveyor belt, but those products are required to be disposed in the individual respective trays in defined arrangements.
In this respect reference may be made for example to EP-A-95-115 548 which, for the above-indicated purpose, discloses a system in which positioned laterally beside a moving conveyor belt for the products are trays which can also be moved, wherein products can be transposed from the conveyor belt into the trays by means of one or more packaging robots which are disposed beside the conveyor belt and which at the free end of their arm have a gripper or a so-called picker, in the form of a vacuum suction device. In order therefore to be able previously to notify the robot of the access position on the conveyor belt, a line camera is arranged upstream of the robots in the direction of movement of the conveyor belt, and above the conveyor belt. The camera is operable to establish along a scanning line extending transversely to the direction of transportation movement of the conveyor belt and over the entire width thereof, whether a product is passing through beneath the camera, while the camera can possibly also determine the rotational orientation of the detected product. In conjunction with information about the speed of movement of the conveyor belt and the spacing between the scanning line and the position of the robot, the required access position and the respectively associated time for gripping a given product on the conveyor belt can then be communicated by means of a computer to the robots which operate at a position downstream of the line camera.
When the conveyor belt is for example about 1.5 meters or 2 meters in width, a line camera of that kind is suspended at a distance of about 1 meter above the conveyor belt. The result of this is that the light beams which scan or sense the outermost edges of the conveyor belt are then very far from being in a condition of extending parallel to each other, but meet the ocular or lens of the camera at an angle of for example 90.degree..
That gives rise to optical distortion effects and parallax phenomena and means on the one hand that each time that the camera is re-adjusted, re-focussed again, a calibration operation has to be performed again, that is to say a test run, during which the machine has to be shut down and cannot operate in its normal mode.
A further disadvantage in this respect is that, particularly when dealing with relatively high products which are disposed at a small spacing relative to each other on the conveyor belt, the fact that the beams impinge at an inclined angle means that the lower edge of a product can be masked or covered from the point of view of the camera by the adjacent upward edge of another product, with the result that those two products cannot be detected as being two separate products. That will result in malfunctions on the part of the machine.
Furthermore a line camera of that kind must have a sensor with a relatively strong resolution power, in most cases a CCD-sensor, as, when dealing with a conveyor belt which is 2 meters in width, it must be possible to achieve reasonably fine scanning in a for example 0.5 mm scanning pattern or raster. That means that 4,000 pixels must be provided in a small area on the CCD-sensor of the camera in the transverse direction of the conveyor belt, and that also makes line cameras of that kind relatively expensive.